1. Field of the Invention
The present invention relates to systems and methods for the guidance and control of vehicles, and in particular to a system and method for mitigating the effect of angular acceleration on optical sensor data.
2. Description of the Related Art
As is described in U.S. Pat. No. 6,285,927, which patent is hereby incorporated by reference, optical sensors such as star trackers are commonly used to assist in the navigation and control of spacecraft. Typically, star trackers report the position of an observed star in a two-dimensional star tracker reference frame, for example, indicating the star position in terms of a horizontal and vertical displacement (H and V, respectively) from the boresight of the star tracker. Such measurements are obtained with charge coupled device (CCD) or similar arrays that collect incident radiation (e.g. photons) over an integration period. The star trackers typically report the centroid of the measurements taken over the integration period and the center time of the integration. Star tracker measurements are sufficiently accurate if the attitude of the spacecraft remains reasonably constant over time, as centroid of the apparent motion of the stars over the integration time will be close to the actual position of the star. Star tracker measurements are also reasonably accurate when the spacecraft experiences a constant angular velocity ω over the integration time, as the centroid of the apparent motion of the star over the integration time will also be close to the actual position of the star at the center-time of the integration.
However, the measured centroid is shifted from the true position of the star at the center time of the integration when the apparent velocity of stars crossing the star tracker field of view changes during the integration period. Star trackers can meet angular accuracy requirements if the angular acceleration of the star tracker is small (e.g. less than 0.1 deg/sec/sec.), but for accelerations above that, the star tracker data includes a bias error that corrupts the spacecraft attitude estimate.
This problem is especially troublesome for agile spacecraft (spacecraft operating with potentially high angular dynamics) applications, where tight pointing requirements and relatively large angular accelerations are the norm.
What is needed is a system and method for improving the accuracy of optical sensor data. The present invention satisfies that need for a wide variety of vehicles and optical sensors, and is particularly useful for star tracker data used to control agile spacecraft having tight pointing requirements.